refactor(toc): cache navigable structure per book (#3869)

* refactor(toc): cache TOC + section fragments per book

Moves the TOC regrouping and section-fragment computation out of
foliate-js/epub.js #updateSubItems into the readest client as
computeBookNav / hydrateBookNav in utils/toc.ts. The result is
persisted to Books/{hash}/nav.json — capturing the book's full
navigable structure (TOC hierarchy + sections with hierarchical
fragments). Compute once, persist locally, hydrate on subsequent
opens. Designed to serve current human-facing navigation (TOC
sidebar, progress math) and future agentic navigation (LLM-driven
seeking by structural location).

Versioned by BOOK_NAV_VERSION for forward invalidation. Existing
books regenerate transparently on next open.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* chore: update worktree scripts

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/readest-app/src/store/readerStore.ts

@@ -13,7 +13,7 @@ import { Insets } from '@/types/misc';
 import { EnvConfigType } from '@/services/environment';
 import { FoliateView } from '@/types/view';
 import { DocumentLoader, TOCItem } from '@/libs/document';
-import { updateToc } from '@/utils/toc';
+import { BOOK_NAV_VERSION, computeBookNav, hydrateBookNav, updateToc } from '@/utils/toc';
 import { formatTitle, getMetadataHash, getPrimaryLanguage } from '@/utils/book';
 import { getBaseFilename } from '@/utils/path';
 import { SUPPORTED_LANGNAMES } from '@/services/constants';
@@ -181,6 +181,21 @@ export const useReaderStore = create<ReaderStore>((set, get) => ({
       }
       // Filter out invalid booknotes
       config.booknotes = config.booknotes?.filter((booknote) => booknote.cfi) ?? [];
+      // Load cached book navigation (TOC + section fragments) or compute and persist.
+      if (book.format === 'EPUB' && bookDoc.rendition?.layout !== 'pre-paginated') {
+        const cachedNav = await appService.loadBookNav(book);
+        if (cachedNav?.version === BOOK_NAV_VERSION) {
+          hydrateBookNav(bookDoc, cachedNav);
+        } else {
+          const freshNav = await computeBookNav(bookDoc);
+          hydrateBookNav(bookDoc, freshNav);
+          try {
+            await appService.saveBookNav(book, freshNav);
+          } catch (e) {
+            console.warn('Failed to persist book nav cache:', e);
+          }
+        }
+      }
       await updateToc(
         bookDoc,
         config.viewSettings?.sortedTOC ?? false,